The present invention relates to implantable stimulation devices, and, more particularly, to electrode arrays for stimulation of the cochlea. Electrode arrays consist of electrode contacts generally placed a long one side of an elongate carrier so that when the array is implanted within one of the cochlear ducts such as the scala tympani, the electrode contacts are positioned in close proximity to the cells that are to be stimulated, allowing such cells to be stimulated with minimal power consumption.
For purposes of clarity, as used herein, an implantable stimulating lead is a device that has one or more electrode contacts that deliver current to tissue to be stimulated. An electrode contact is that part of the stimulating device which is actually electrically conductive and is in contact with the body tissue that is to be stimulated. The term “electrode” may sometimes be used narrowly as the electrode contact or contacts only and, other times, more broadly, as the electrode contact or contacts and all the surrounding structure, including the insulation carrier that the contacts are placed upon, as well as the conductor wires and any other assemblies within or on the insulation carrier. As used herein, when the term “electrode” is used it will include the electrode contacts and all surrounding structures. In addition, when the term “lead” is used, it will refer to the entire elongate device and include that portion of the lead which is the “electrode.” The term “electrode array” will refer to that portion of the lead that includes all of the electrode contacts and the immediate structures upon which the electrode contacts are attached.
Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Of these, conductive hearing loss occurs where the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, from damage to the ossicles. Conductive hearing loss may often be helped by using conventional hearing aids that amplify sounds so that acoustic information can reach the cochlea and the hair cells. Some types of conductive hearing loss are also amenable to alleviation by surgical procedures.
In many people who are profoundly deaf, however, the reason for their deafness is sensorineural hearing loss. This type of hearing loss can arise from the absence or the destruction of the hair cells in the cochlea which transduce acoustic signals into auditory nerve impulses. Individuals with sensorineural hearing loss are unable to derive any benefit from conventional hearing aid systems no matter how loud the acoustic stimulus is, because the mechanism for transducing sound energy into auditory nerve impulses has been damaged. Thus, in the absence of properly functioning hair cells, auditory nerve impulses cannot be generated directly from sounds.
To overcome sensorineural deafness, cochlear implant systems or cochlear prostheses have been developed, which can bypass the hair cells located in the vicinity of the radially outer wall of the cochlea by presenting electrical stimulation to the auditory nerve fibers directly. This leads to the perception of sound in the brain and provides at least partial restoration of hearing function. Thus, most of these cochlear prosthesis systems treat sensorineural deficit by stimulating the ganglion cells in the cochlea directly using an implanted electrode or lead that has an electrode array.
A cochlear prosthesis operates by directly stimulating the auditory nerve cells, bypassing the defective cochlear hair cells that normally transduce acoustic energy into electrical activity to the connected auditory nerve cells. In addition to stimulating the nerve cells, the electronic circuitry and the electrode array of the cochlear prosthesis separate the acoustic signal into a number of parallel channels of information, each representing a narrow band of frequencies within the perceived audio spectrum. Ideally, each channel of information should be conveyed selectively to a subset of auditory nerve cells that normally transmits information about that frequency band to the brain. Those nerve cells are arranged in an orderly tonotopic sequence, from the highest frequencies at the basal end of the cochlear spiral to progressively lower frequencies towards the apex. In practice, however, this goal can be difficult to realize because of the particular anatomy of the cochlea.
Over the past several years, a consensus has generally emerged that the scala tympani, one of the three parallel ducts that make up the spiral-shaped cochlea, provides the best location for implantation of an electrode array used as part of a cochlear prosthesis. The electrode array to be implanted in the scala tympani typically can comprise a thin, elongated, flexible carrier containing several longitudinally disposed and separately connected stimulating electrode contacts, conventionally numbering about 6 to 30. Such an electrode array is pushed into the scala tympani duct in the cochlea to a depth of about 20-30 mm via a surgical opening made in the round window at the basal end of the duct.
In use, the cochlear electrode array delivers electrical current into the fluids and tissues immediately surrounding the individual electrode contacts to create transient potential gradients that, if sufficiently strong, cause the nearby auditory nerve fibers to generate action potentials. The auditory nerve fibers branch from cell bodies located in the spiral ganglion which lies in the bone or modiolus, adjacent to the inside wall of the scala tympani. Because the density of electrical current flowing through volume conductors such as tissues and fluids tends to be highest near the electrode contact that is the source of such current stimulation, one electrode contact site tends to selectively activate those spiral ganglion cells and their auditory nerve fibers that are closest to that contact site. Thus, it is important generally for the electrode contacts to be positioned as close to the ganglion cells as possible. Conventionally, after implant, the electrode array consisting of electrode contacts should hug the modiolar wall (or inside wall of the scala tympani). When the electrode side of the array is positioned closest to the modiolar wall, the electrode contacts are on the medial side of the lead.
In order to address the above need, it is known in the art to make an intracochlear electrode array that includes a spiral-shaped, resilient carrier which generally has a natural, spiral shape so that the array better conforms to the shape of the scala tympani. See, e.g., U.S. Pat. No. 4,819,647, which is incorporated herein by reference. While the electrode array with a spiral-shaped carrier shown in the '647 patent represents a significant advance in the art, it lacks sufficient shape memory to allow it to return to its original curvature (once having been straightened for initial insertion) and to provide sufficient hugging force to wrap snugly against the modiolus of the cochlea.
It is also known in the art, as shown in U.S. Pat. Nos. 5,545,219 and 5,645,585, to construct an electrode carrier from two initially straight members, a rod-like electrode carrier and a flexible rod-like positioning member. The '219 and '585 U.S. patents are also incorporated herein by reference. As shown in these patents, the two members extend in substantially parallel relation to and closely alongside each other, but are connected to each other only at their respective leading and trailing ends. After implant, a pushing force is applied to the positioning member so that it is forced to assume an outwardly arched configuration relative to the electrode carrier, thereby forcing the electrode carrier into a close hugging engagement with the modiolus and placing the electrode contacts as close to the cells of the spiral ganglion as possible. The '219 patent, in particular, shows in FIGS. 1-10 and describes in the accompanying text an excellent summary of prior art electrodes and the deficiencies associated therewith. Other patents relevant to the subject matter of cochlear stimulation leads are: U.S. Pat. Nos. 6,125,302; 6,070,105; 6,038,484; 6,144,883; and 6,119,044, which are all herein incorporated by reference.
While the electrode arrays taught in the above-referenced '219 and '585 patents are based on the correct goal, i.e., to force the electrode carrier into a close hugging engagement with the modiolus, it does so only by using an additional element that makes manufacture of the lead more difficult and expensive and only by applying an additional pushing force to an electrode structure after it has already been inserted into the cochlea. Such additional pushing force may cause damage to the delicate scala tympani or cause the electrode contacts to twist or to separate away from the modiolus, rather than be placed in the desired hugging relationship.
Thus, while it has long been known that an enhanced performance of a cochlear electrode or lead can be achieved by proper placement of the electrode contacts close to the modiolar wall of the cochlea, a major challenge has been obtaining a electrode/lead design that does not use excessive force to achieve this close placement. There has either been the need for application of an external and possibly unsafe force or a lack of sufficient shape memory to allow the electrode to assume or return to the desired curvature (after the electrode has been initially straightened during insertion) so that the electrode array wraps snugly around the modiolus of the cochlea.
It is thus evident that improvements are still needed to obtain a cochlear electrode that has shape memory, is easily implanted so that excessive force is not required, and can be easily manufactured.